1. Field of the Invention
The present invention relates to films that exhibit a superior balance of physical properties. The films are produced with polyethylenes having a broad composition distribution (CD) and molecular weight distribution (MWD).
2. Description of the Related Art
Metallocene-catalyzed ethylene polymers are known to produce tough films measured in terms of dart drop impact strength (dart). Conventional Ziegler-Natta catalyzed linear low density polyethylenes (Z-N LLDPE) are known to have good processability, stiffness and tear strength, as measured by extruder pressures and motor load, 1% secant modulus, and Elmendorf tear strength. Ideally, a polyethylene resin would have Z-N LLDPE processability and produce a film exhibiting a combination of metallocene like toughness and Ziegler-Natta like stiffness and tear strength. It is possible to improve the toughness of films (e.g. MD tensile strength) by increasing the amount of orientation in the machine direction during film blowing. However, conventional knowledge in the polyethylene film art says that by increasing the machine direction (MD) orientation in films during manufacture of these films, other physical properties, such as MD tear strength, will decrease.
To this point, in Polymer Engineering and Science, mid-October 1994, vol. 34, No. 19, the disclosure of which is incorporated herein by reference, the authors discuss processing structure properties relationships in polyethylene blown film. The authors suggest that MD Elmendorf tear is found to be inversely related to drawdown ratio and MD shrinkage.
Further, in Polymer, 41 (2000) 9205-9217, the disclosure of which is incorporated herein by reference, the authors suggest that at high MD extension rates, a greater number of molecules will be oriented along the MD prior to the onset of crystallization, and that this is detrimental from a MD tear performance perspective.
Metallocene catalyst components can be combined to form blend compositions as described in PCT publication WO 90/03414 published Apr. 5, 1990, the disclosure of which is incorporated herein by reference. Also mixed metallocenes as described in U.S. Pat. Nos. 4,937,299 and 4,935,474, the disclosure of both which are incorporated herein by reference, can be used to produce polymers having a broad molecular weight distribution and/or a multimodal molecular weight distribution.
U.S. Pat. No. 5,514,455 suggests that a reduction in gauge of polyethylene films results in an increase in tear values. This document employs a titanium magnesium catalyst for polyethylene production and includes titanium residues in the polyethylene. Reported values of Elmendorf machine direction (MD) tear to transverse direction (TD) tear, are in the range of 0.1-0.3 for inventive examples.
U.S. Pat. No. 5,744,551, the disclosure of which is incorporated herein by reference, suggests a balance of tear property improvement. This document also employs a titanium magnesium catalyst for polyethylene production and includes titanium residues in the polyethylene. Further, the MD/TD tear ratios are in the range of 0.63-0.80 for inventive examples.
U.S. Pat. No. 5,382,630, the disclosure of which is incorporated herein by reference, discloses linear ethylene interpolymer blends made from components that can have the same molecular weight but different comonomer contents, or the same comonomer contents but different molecular weights, or comonomer contents which increase with molecular weight. U.S. Pat. No. 5,382,630 suggests multimodal polyethylene blends for which tear strength can be controlled. However, this document uses only intrinsic tear, and is silent on Elmendorf MD/TD tear ratios and on any other values but intrinsic tear.
Also, in U.S. Pat. No. 6,242,545 and U.S. Pat. No. 6,248,845 as well as provisional applications U.S. Ser. No. 60/306,503 filed Jul. 19, 2001 and 60/306,903 filed Jul. 19, 2001, the disclosures of all which are incorporated herein by reference, the patentees/applicants of these documents reported production of either broad composition distribution, narrow molecular weight, or broad composition distribution, relatively broad molecular weight distribution polyethylenes. However, these documents show an improvement in cast film MD tear, but no appreciable improvement for blown film.
There is a need, therefore, for a polyolefin film, more specifically a blown polyethylene film, that has high machine direction tear (MD tear) and/or high transverse direction tear (TD tear), and/or high dart drop impact resistance (dart), made from a polyethylene that is easier to process than prior metallocene catalyst produced linear low density polyethylenes (mLLDPE). In other words it is desirable to have the processability, stiffness and tear strength of a ZN-LLDPE combined with the dart impact strength of a mLLDPE.